1. Field of the Invention
The present invention relates generally to backing (bulk, auxiliary or secondary) storage devices and more particularly to semiconductor backing storage devices incorporated with a computer system or a multicomputer system. Generally, the backing storage device can store a great capacity of information data, but has shortcomings such that the access time is slow when compared with that of the main (internal or immediate access) memory arranged within the computer system.
2. Description of the Prior Art
Conventionally, magnetic disc storage devices are usually incorporated with a computer system as backing storage memory. This is because the magnetic disc storage devices are of random access storage which can store a great capacity of information data. Here, the random access means direct access in which any locations designated by addresses can be accessed (data are searched) within the same access time irrespective of the preceding access address.
The magnetic disc storage device is made up of a plurality of discs rotating at a constant speed and a series of magnetic arms each provided with one or more reading/writing heads. Magnetic paint material is applied on both the surfaces of the disc. Information data are magnetically written in or read out of the disc surface by bringing the arm head near the surface of the rotating disc. The arm head can be moved to any required areas (recording tracks) of the disc surface in accordance with an instruction. Each disc surface has a great number of recording tracks, and the tracks are further divided into a great number of sections or sectors. In access to the above-mentioned magnetic disc device, it inevitably takes an access time of approximately 40 milliseconds. This is because there exist a waiting time (latency) of the disc, (from the time when a data transfer command signal is output to the time when a designated sector on a rotating disc comes near the head) and a head seek time (from the time when a data transfer command signal is output to the time when the head moved radially with respect to the disc comes near a designated track of a rotating disc). The access time of the magnetic disc is much longer than that of the main memory arranged within a computer system.
Therefore, in the case where a computer system connected to a magnetic disc backing storage device is used for a process control system, for instance, which requires a high response speed, although a great capacity of information data can be transferred, since the access time is longer, it is impossible to efficiently control the process control system by the computer. In such a case as described above, another computer system provided with a magnetic disc backing storage device should be combined with the process control system to transfer necessary information data from the control systems to the magnetic disc devices in idle time, thus resulting in problems in that another computer system should additionally be arranged and further anther means for combining the process control system with the computer systems is additionally required.
Further, the difference in access time between computer main memory and magnetic disc backing storage device causes mismatching in data transfer; that is, data transferred from the computer to the backing storage are not well synchronized with those transferred from the magnetic disc backing storage to the computer. To overcome the above-mentioned drawbacks, in general FIFO (first-in first-out) buffers having a capacity of several words (bytes) are disposed in the interface between the computer and the backing storage. However, these buffers will not reduce the access time itself of the magnetic disc backing storage, but only attains the matching or synchronization between the two.
On the other hand, multicomputer systems are well known. The system is made up of a plurality of computers, in which the main computer executes various complex operations or calculations while other computers execute input/output control or routine jobs under the supervisory control of the main computer. To link plural computers, that is, to transfer information data between or among the computers, communication control systems, shared memory systems and cross-call magnetic disc systems have conventionally been utilized. In the case of the computer link based on a communication control system, it is impossible to transfer data at high speed, because there exists a long data transferring time or overhead times (indirect times required for decisions) in communication protocol (software procedures).
In the case of computer link based on the shared memory system, since a part of the main memory arranged within the main computer is often used as the shared memory in order to facilitate the operations, it is impossible to increase the capacity of the shared memory infinitely because the memory capacity in the main computer is not limitless.
In the case of cross-call magnetic disc devices, although the storage capacity can be increased almost limitlessly, since a relatively long access time of approximately 40 milliseconds is required due to the waiting time of the rotating magnetic discs and head seek time of the shifted arm head, it is impossible to transfer information data at high access speed.